The invention relates to an apparatus for supplying power to a field device, such as an actuator or motor.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
In the case of field devices of all types, including converters, motor starters, input/output modules, it is necessary to have to communicate with the devices, for example, via a field bus even in the case in which the line voltage supply is switched off. This is particularly necessary for bringing the devices into operation, but also for diagnostic purposes during operation. When they are brought into operation, the field devices are not constantly connected to the line voltage supply. As a result, these field devices cannot be supplied from the system voltage.
European Pat. No. EP 0 593 995 Al discloses an apparatus, in the case of which field devices, for example converter-fed motors in a conveying system, are connected to a central control cabinet by means of a multiphase electrical power line, a DC supply line and a data bus. A multi-pole switching contactor, a DC voltage supply device and a control system are accommodated in this central control cabinet. The multi-pole switching contactor is connected to a power supply system on the system side. In the field devices, all three types of line are looped through, i.e. the lines are therefore not interrupted at any point, but the required currents or data are merely tapped off.
This variant, in which an auxiliary voltage is brought to the field devices by means of a parallel cable, has the following advantages:                complete disconnection of the line voltage is possible,        as a result of which:                    the field device can be operated in a hazard-free manner given the available communication capability,            the risk when the field device is brought into operation is reduced and, as a result,            it is possible for the remote drive system to be brought to a standstill safely.                        
However, this apparatus for supplying power to at least one field device also has disadvantages:                Additional cable having a large cross section (owing to the voltage drop along the cable) results in additional system costs.        In the case of the additional cable with a small cross section, an intermediate supply of the auxiliary voltage is required in the panel.        Additional plug-in connectors (at least terminals and bushings) are required which represent a considerable cost factor, particularly in the case of devices having a high degree of protection, primarily if it is also necessary for the auxiliary voltage to be looped through.        Considerable extra complexity is required in the field device.        
It would be desirable to modify the prior art apparatus which for supplies power to at least one field device in such a way that a separate power cable for the DC supply is no longer required.